1. Field of the Invention
The current invention concerns a system for imaging simultaneously multiple layers within a three-dimensional object field and has applicability in fields including optical information storage, imaging short-timescale phenomena, microscopy, imaging three-dimensional object structures, passive ranging, laser beam profiling, wavefront analysis and millimeter wave optics.
2. Discussion of Prior Art
The use of an undistorted amplitude grating to produce identical images of a scene in several diffraction orders is known. Most of the energy is concentrated in the zero order with most of the remaining energy being contained in the +1 and −1 orders. Phase or phase and amplitude gratings may be used to change the distribution of energy in the different diffraction orders.
It is also known that distortions of such a grating (i.e. dislocations in a direction perpendicular to the grating lines) may be used to produce phase changes in the optical system and thus shape the wavefront in the back focal plane of the system. This effect has been used to separate redundant baselines in a masked-aperture system using a dislocated grating and has formed the basis for computer generated holograms for many years (P M Blanchard, A H Greenaway, R N Anderton, R Appleby, ‘Phase calibration of arrays at optical and millimeter wavelengths’, J. Opt. Soc. Am. A., Vol 13, No. 7, pp1593-1600, 1996; G Tricoles, ‘Computer generated holograms: an historic review’, Appl. Opt., Vol 26, No. 20, pp4351-4360, 1987 and M Li, A Larsson, N Eriksson, M Hagberg, Continuous-level phase only computer generated holograms realised by dislocated binary gratings’, Opt. Lett., Vol. 21, No 18, pp1516-1518, 1996).
The imaging of a three-dimensional object using a ‘through-focal series’ is also known. By this method a sequence of images of the object are taken with the optical system focused on different planes in the object field. An alternative approach forms simultaneously a matrix of images recorded through a matrix of lenses, each of which provides a different focus condition.
A disadvantage of the ‘through-focal series’ is that because the images are recorded sequentially it is ill-suited to imaging the three-dimensional structure of dynamic processes. A disadvantage of the second approach is its complex design and that the resolution obtained is limited to the resolution delivered by the individual lenses in the array, the diameter of each of which (thus image resolution) is constrained by the space into which the array may be packed.
The storage of data in three dimensional, optically readable, storage medium is also known (S Jutamulia and G M Stori, ‘Three-Dimensional Optical Digital Memory’, Optoelectronics—Devices and Technologies Vol 10, No. 3, pp343-360, 1995 and K Kobayashi and S S Kano, ‘Multi-Layered Optical Storage with Nonlinear Read/Write’, Optical Review, Vol 2, No 1, pp20-23, 1995). These papers review the media and architecture for various three dimensional optical memories.
In a high performance, near diffraction limited optical system such as a compact disk player, all sources of wavefront aberrations must be considered. In a standard compact disk, the data layer is covered with a substrate several hundred microns thick. Propagation of light through this substrate (which is essentially a parallel plate) introduces spherical aberration, increasing the spot size on the data layer and degrading resolution. This effect is overcome in conventional, single layer, compact disk systems by building spherical aberration correction into the objective lens.
In a multi-layer optical data storage medium the degree of spherical aberration is dependent on the depth of the data layer in the storage medium, hence when reading from each distinct layer a different level of spherical aberration correction is required. An aberration corrected objective lens is therefore not sufficient. Several patents on multi-layer optical data storage systems, which rely on a moving lens to focus at different depths, have suggested ways of performing ‘active’ spherical aberration correction. U.S. Pat. No. 5,202,875 suggests using a stepped block of substrate material which is moved across the optical beam (using a voice coil motor) to a position dependent on the layer being read, such that the thickness of material that the beam passes through is constant. Other suggestions include a pair of prisms, one of which is translated, a rotating disk of variable thickness and movable compensation plates.
All of these approaches introduce additional moving parts and complexity into the system.